by Tatyana Woodall
新研究首度探討從熱帶地球最高的山脈的頂峰採集而來的冰芯，其中的新證據為亞馬遜盆地過去60年的氣候紀錄提供了獨一無二的觀點。
這張近照詳細呈現了祕魯瓦斯卡蘭山上的大量冰雪。圖片來源：Getty Images
瓦斯卡蘭山(Nevado
Huascarán)坐落在祕魯安地斯山脈的中部，這座熱帶高山的冰河保存了整個地區的氣候歷史。研究人員長久以來都對此地有濃厚的興趣，因為不同於從極區採集到的冰芯，熱帶的冰芯樣品可以對聖嬰、季風等現象提供豐富的資訊。
俄亥俄州立大學的研究人員發表在《地球物理研究期刊：大氣》(JGR
Atmospheres)的研究包含了四根冰芯樣品，其中兩根採自於鞍部，也就是兩座山脊之間的最低點；另外兩根則是首次從海拔7000公尺左右的山頂採集而來。
研究人員比較瓦斯卡蘭山不同海拔的冰河中保存的穩定氧同位素紀錄。通常研究冰芯的科學家把同位素當作溫度隨時間變化的代用指標，但是熱帶地區同位素紀錄的解析結果可能牽扯到更加複雜的作用。
他們發現同位素紀錄與太平洋的表面溫度以及熱帶南美的雨量在統計上都有顯著關係。此外，他們也發現比起高度較低的地方，瓦斯卡蘭山山頂的穩定氧同位素更容易受到熱帶太平洋海水表面溫度的大尺度變化影響。
此發現的重要之處在於它指出依據所處的高度，穩定氧同位素紀錄下來的氣候史可能代表了不同機制，或是呈現出該區域不同面向的故事。整體來看，他們的結果指出熱帶太平洋的氣候對於山頂同位素紀錄的影響有增加的趨勢，可能是因為最近幾十年觀測中的氣候變遷速率也越來越快。
「從古氣候的觀點來看，數據告訴我們這些冰芯對於探討熱帶聖嬰現象的歷史可能很有用，」研究主要作者Austin
Weber表示。他是俄亥俄州立大學伯德極地與氣候研究中心的博士生。「而我們對這方面的歷史真的不甚瞭解，因為熱帶地區的觀測數據和歷史紀錄都沒有很多。」
Weber表示聖嬰年是指赤道太平洋表面海水溫度異常升高的年份，此時信風的強度會下降，造成亞馬遜盆地的雨量變少。「如果降水變少，同位素的分化過程就會跟平常不一樣，」他說。
Weber表示眾所皆知溫度和降水是影響冰河大小的主因。但是在過去60年，因為溫度急遽升高的緣故，使得瓦斯卡蘭山的冰河快速後退。
伯德極地與氣候研究中心的資深研究員Lonnie
Thompson是研究共同作者，他說考慮到在瞭解地球錯綜複雜的海洋—大氣系統時，熱帶的冰芯樣品可以做為重要的代用指標，瓦斯卡蘭山的冰河快速後退是個令人痛心的消息。
「這些冰芯的美妙之處在於它們可以讓你看見人類開始改變氣候系統之前，自然情況下氣候是如何變化，」Thompson表示。他也是俄亥俄州立大學地球科學系的特聘教授。
數十年來，由於雪崩以及藏在雪中的冰河裂隙帶來的風險，完全沒有研究考察隊伍到達瓦斯卡蘭山山頂來收集這些古代的氣候紀錄。直到2019年七月，Thompson和他的團隊才成功找到一條路線通往瓦斯卡蘭山的南峰。團隊在海拔6050公尺的鞍部以及海拔6768公尺的山頂各鑽出兩根直達岩床的冰芯，總計取出了471公尺長的冰河冰芯樣品。
「從瓦斯卡蘭山取得冰芯非常困難且危險，未來可能不會再有實地考察的機會，」Thompson表示。「此文章是關於這組冰芯第一個發表的研究結果，我相信之後還會有一系列相關文章。而這組冰芯可能會是我整個研究生涯採集過最獨特的樣品。」
本研究的經費來自美國國家科學基金會。其他共同作者包括Mary
Davis、Ellen
Mosley-Thompson、 Emilie
Beaudon、Don
Kenny、Ping-Nan
Lin、Roxana
Sierra-Hernández，皆為俄亥俄州立大學伯德極地與氣候研究中心的研究人員。
Ice cores from Earth’s highest
tropical peak provide insight into climate variability
In the first study to examine ice cores
from the summit of the highest tropical mountain in the world, new evidence
provides unique insight into the climate record of the Amazon Basin over the
last six decades.
Nestled within the central Peruvian Andes lies Nevado
Huascarán, a tropical mountain whose glaciers preserve the climate histories of
the entire region. Researchers have long been interested in studying this area,
because unlike ice cores recovered from the poles, core samples taken from
tropical areas of the world can reveal a wealth of information about phenomena
like El Niño and seasonal monsoons.
The study, by researchers from The Ohio State University
and published in JGR Atmospheres,
involves four ice core samples – two from mountain col, which is the lowest
point between two ridges, and for the first time, two from the summit, nearly
7,000 meters above sea level.
The researchers compared the oxygen-stable isotope
records preserved in glacial ice at these different elevations on the mountain.
Scientists who study ice cores use isotopes as a proxy for temperature change
over time, but in tropical regions interpreting the isotope records can be a
more complex process.
Their findings showed that the isotope records share
a statistically significant relationship with sea surface temperatures in the
Pacific and with rainfall over tropical South America. The oxygen-stable
isotopes from the summit were also found to be more sensitive to large-scale
changes in tropical Pacific sea surface temperature than the ones found at the
lower levels of the mountain.
This is significant because it suggests that
depending on the elevation, past climate histories recorded by the
oxygen-stable isotopes may reveal different mechanisms or stories about the
region. Overall, their results suggest that the influence of tropical Pacific
climate on the summit isotope records is increasing, likely due to the rapid
rates of climate change observed in recent decades.
“From a paleo-climate perspective, the data tells us
that these cores may be useful for looking at the history of El Niño in the
tropics,” said Austin Weber, lead author of the study and a PhD student at the
Byrd Polar and Climate Research Center at Ohio State. “And we don’t really have
very good histories of that because there are not many observational data sets
or historical records for the tropics.”
During the years designated El Niño – when sea
surface temperatures in the equatorial Pacific are anomalously warm – trade
winds weaken and cause less rainfall to occur in the Amazon Basin, said Weber.
“And if there’s less precipitation, the isotopes are going to fractionate
differently than normal,” he said.
It’s well known that temperature and precipitation
amount are major factors that affect the size of glaciers, Weber said. But,
over the past 60 years, temperatures have been rising significantly, pushing
the ice on Nevado Huascarán into an accelerated retreat.
This is devastating news, said Lonnie Thompson,
co-author of the study and a senior research scientist at the Byrd Polar and
Climate Research Center, considering that tropical ice core samples can act as
important proxies for understanding Earth’s intricate ocean-atmosphere system.
“The beauty of these ice cores is that they give you
a perspective into what the natural variability was before humans started
altering the climate system,” said Thompson, who is also a distinguished
university professor of earth sciences.
Yet for decades, due to the risk of avalanches and
hidden snow-lined crevasses, no research expedition had ever been able to reach
the peak of Nevado Huascarán to collect these ancient records. That is, until
July of 2019, when Thompson and his team successfully navigated their way to
the summit of the mountain’s South Peak. The team recovered two ice cores to
bedrock from the col drill site – which is 6,050 meters above sea level – and
two cores to bedrock from the summit at 6,768 meters above sea level –
recovering 471 meters of glacial ice cores in total.
“It’s a very difficult and dangerous place to recover
ice cores and that expedition probably won’t be repeated,” said Thompson. “This
paper is the first research to be published of what I believe will be a whole
series of papers on what are probably the most unique collection of ice cores
collected in my entire career.”
This work was supported by the National Science
Foundation. Other co-authors were Mary Davis, Ellen Mosley-Thompson, Emilie
Beaudon, Don Kenny, Ping-Nan Lin and Roxana Sierra-Hernández, all of Ohio
State’s Byrd Polar & Climate Research Center.
原始論文：A. M. Weber,
L. G. Thompson, M. Davis, E. Mosley‐Thompson, E. Beaudon, D. Kenny, P.‐N. Lin, R.
Sierra‐Hernández. Drivers
of δ18O Variability Preserved in Ice Cores From Earth's Highest
Tropical Mountain. Journal of Geophysical Research: Atmospheres,
2023; 128 (19) DOI: 10.1029/2023JD039006
引用自：Ohio State University. "Ice cores from
Earth's highest tropical peak provide insight into climate variability."
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